After Windows has shut down, USB ports still hv power?

I have two computers at home, both running Windows XP Pro.

CompA - Dell Dimension L700r
CompB - P4/3.0GHz/GA-8IPE1000-G

I have an optical mouse. There are 2 LEDs in the mouse. The interface between the mouse and computer is as follows:

Mouse -> USB to PS/2 Adapter -> Computer PS/2 Port.

I found out that AFTER Windows has shut down, the LEDs in the mouse, when it's connected to CompB, were still ON. For the case in which the mouse was connected to CompA, the LEDs in the mouse were OFF AFTER the Windows has shut down.

Besides, I have a 4-port USB Hub connected to CompB. The hub has few LEDs. After Windows has shut down, all the LEDs were still ON. After I powered off the MAIN switch, then all the LEDs were OFF.

Why, for CompB, the USB ports still have power even after Windows has shut down? Pls advise. Thanks.

Most desktops have powered usb even when the computer is switched off. For notebooks, they are often the opposite case to conserve battery power.